1. Field of the Invention
The present invention relates to a control circuit for a power supply, and more specifically relates to a switching controller for a switching mode power supply.
2. Description of Related Art
Power supplies have been widely used to provide regulated output voltages. For safety, an off-line power supply must provide galvanic isolation between its primary side and secondary side. An optical-coupler and a secondary-side regulator are needed to regulate the output voltage of the off-line power supply. In order to reduce the device counts and do without a secondary-side feedback circuit, a primary-side control technique has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter”, issued to Randolph D. W. Shelly. However, foregoing prior art cannot measure an accuracy voltage signal from the transformer. This shortcoming is the main object of the present invention to overcome.